Coal is one of the most bountiful sources of fuel in the world. Coal is typically found as a dark brown to black graphite-like material that is formed from fossilized plant matter. Coal generally comprises amorphous carbon combined with some organic and inorganic compounds. The quality and type of coal varies from high quality anthracite (i.e., a high carbon content with few volatile impurities and bums with a clean flame) to bituminous (i.e., a high percentage of volatile impurities and bums with a smoky flame) to lignite (i.e., softer than bituminous coal and comprising vegetable matter not as fully converted to carbon and bums with a very smoky flame). Coal is burned in coal-fired plants throughout the world to produce energy in the form of electricity. Over the years it has been recognized that certain impurities in coal can have a significant impact on the types of emissions produced during coal combustion. A particularly troublesome impurity is sulfur. Sulfur can be present in coal from trace amounts up to several percentages by weight (e.g., 7 percent by weight). Sulfur may be found in coal in various forms, e.g., organic sulfur, pyritic sulfur, or sulfate sulfur. When coal containing sulfur is burned, sulfur dioxide (SO2) is typically released into the atmosphere in the combustion gases. The presence of SO2 in the atmosphere has been linked to the formation acid rain, which results from sulfuric or sulfurous acids that form from SO2 and water. Acid rain can damage the environment in a variety of ways, and, in the United States, the Environment Protection Agency (EPA) has set standards for burning coal that restricts SO2 emissions from coal-fired plants.
While coal is produced in the United States in many areas of the country, much of the coal that is easily mined (and therefore inexpensive) often contains high levels of sulfur that result in levels of SO2 in the combustion gases greater than allowed by the EPA. Thus, coal-fired plants often must buy higher quality coal from mines that may be located long distances from the plants and pay significant transportation and other costs. A significant body of technology has been developed over time to reduce the amount of SO2 in combustion gases from burning high sulfur coal. This technology has involved treatments to coal during pre-combustion, during combustion, and during post-combustion. However, such treatments have generally not achieved a satisfactory combination of efficacy in reducing SO2 emissions and economic feasibility in implementation.
It is against this background that a need arose to develop the present invention.